The use of proppants in conjunction with the hydraulic fracturing of shale and other “tight oil” formations has resulted in remarkable petroleum recoveries in the U.S. For example, the annual oil production in North Dakota now exceeds the production from Alaska—a striking development buttressed on the use of proppants to “prop” open the hydraulically-created fractures in the oil-bearing rock. As a result, the proppant market in the U.S. is now in the billions of dollars per year and growing.
The recovery of tight oil through the use of proppants could easily make the U.S. self-sufficient in energy or even a net exporter of petroleum. However, environmental concerns are hampering widespread use of hydraulic fracturing. In that regard, one can appreciate that many of these concerns are overblown in that the rock formations being fractured are typically thousands of feet deep whereas ground water supplies are far removed from such depths. Nevertheless, the growth of hydraulic fracturing techniques depends upon addressing and monitoring any escape of the hydraulic fracturing fluids from the fractured rock formation. For example, such escape is conceivable if a natural fault extends through the hydrocarbon-bearing rock formation being fractured up through the shallower depths from which ground water is extracted.
Existing electronics-based downhole gauges are unable to survive the increasingly high temperature conditions experienced in modern, deep oil and gas wells. Raman-OFDR optical-fiber-based sensors offer many advantages over existing downhole electronic gauges. However, the single string of optical fiber just covers a small area of the entire well volume. The problem becomes more serious for fractured wells in which the use of fiber-optics for in-situ monitoring of the fracking process is impossible.
It is not just hydraulically-fractured wells that require monitoring. For example, the Deepwater Horizon disaster in the Gulf of Mexico involved a compromised well casing. Highly-pressurized natural gas displaced sea water and drilling mud in the riser to the drilling platform and caused an explosion. The drilling platform eventually sank, leading to an environmental disaster of considerable proportions. It is believed that the cement casing failed in the Deepwater well, which led to the natural gas from the penetrated reservoir blasting through the riser to the floating drilling platform.
There is thus a need in the art for better techniques to monitor hydraulically fractured wells. Moreover, there is a need in the art for techniques to monitor the integrity of well casings.